Not applicable.
Not applicable.
Perfluorocarbons (PFCs) have many favorable properties such as good stability, non-flammability, low surface tension and low solubility. These favorable properties make perfluorocarbons useful in many applications such as lubricants, solvents, refrigerants, hydraulic fluids, fire extinguishers, insulators and cleaning agents. In the past, perfluorocarbons were manufactured, used and discharged to the environment freely. However, it was recognized some years ago that their inertness, besides making them materials of choice for the uses above, also causes them to break down extremely slowly in the upper atmosphere, giving them the potential to create global warming.
For example, many processes involved in the fabrication of semiconductors, e.g., etching, deposition, oxidation, are carried out within closed reaction chambers and involve the use of perfluorocarbons. These chambers are also referred to as tools. Typically, silicon wafers that are fabricated into semiconductors are placed into these chambers, or tools, one-by-one, or in some cases, more than one at a time. After a processing step is completed, the silicon wafers are removed from one chamber, usually by robotic means, and passed to the next chamber (or tool) where they undergo the next step in the process. Over time, as a result of these processing steps, the internal walls of the chambers become contaminated as a result of the various gases and other materials that are present in the chamber during the processing steps. Thus, periodically, the internal walls of these chambers must be cleaned. One manner for cleaning these chambers is through the use of perfluorocarbons (PFCs). PFCs are introduced into the chamber for cleaning purposes while the chamber is devoid of silicon wafers and is between processing steps. The cleaning process includes exposing the chamber to PFCs, sometimes in combination with high temperatures. At the end of the cleaning process, an effluent stream containing contaminant material and PFC cleaning agents is removed from the processing chamber by a vacuum pump located downstream of the chamber and pumped away.
These effluent streams can contain mixtures of global warming perfluorocarbon compounds such as C2F6, CF4, NF3, SF6, CHF3, C3H8, CH3F and C2HF5, reactive compounds such as HF, F2, COF2, SiF4 and SiH4, and environmentally benign atmospheric gases such as N2, O2, CO2, H2O and N2O. Many of these reactive compounds result from reaction between the PFCs used in the cleaning process and the contaminants deposited on the walls of the reaction chamber. Typical effluent streams exiting the reaction chamber during cleaning contain approximately 50% PFCs, and 50% nitrogen and other atmospheric gases and flow at a typical rate of 2,000 standard cm3 per minute. The stream is typically under a vacuum at a pressure of approximately 10 torr to 100 torr. Since these effluent streams usually include reactive compounds that can damage the vacuum pump during removal from the chamber, the effluent stream is usually diluted to a proportion of approximately 1% PFCs by mole and approximately 99% N2 and other atmospheric gases prior to entering the vacuum pump. Dilution enables the vacuum pump to operate more efficiently.
Dilution is accomplished by adding approximately 50,000 to 100,000 standard cm3 per minute (50 to 100 standard liters per minute) of N2 to the effluent stream. This heavy dilution with nitrogen minimizes the damaging effects of reactive compounds, and is necessary for efficient operation of the vacuum pump. The diluted stream exits the vacuum pump at an approximate pressure of 760 torr.
Prior to atmospheric venting, the vacuum pump effluent stream must be treated. Environmental considerations require removal of damaging compounds from the semiconductor effluent stream prior to venting to the atmosphere. However, efficient separation of the low quantities of damaging compounds, e.g., 1%, from the comparatively high quantities, e.g., approximately 99%, of environmentally benign nitrogen and other atmospheric gases becomes more difficult after the dilution step. Corrosive components can be removed from these effluent streams using well established scrubbing technology. Reactive components, such as SiH4, can be removed using combustion into less harmful compounds. However, until recently, PFC compounds were frequently vented without emission control. Recent efforts to reduce emission of these global warming compounds have led to the use of emission control methods such as adsorption, membrane separation or cryogenic distillation technologies and devices for carrying out these methods. Such emission control methods and devices provide means to separate the PFCs from the atmospheric gases (e.g., N2). As shown in the sequence below, these emission control devices are typically located after the corrosive gas scrubber, at which point atmospheric gases typically comprise approximately 99% of the mixture, the balance consisting largely of PFC compounds.
REACTION CHAMBER (TOOL)xe2x86x92VACUUM PUMPxe2x86x92SCRUBBERxe2x86x92(99% Atmospheric Gases, 1% PFCS)xe2x86x92EMISSION CONTROL DEVICE (Adsorption Device or Membrane Separation Device)xe2x86x92VENT.
As a typical example, a single-stage membrane separation device can provide a PFC-enriched mixture containing approximately 80% to 95% atmospheric gases. Re-use of the PFCs requires additional steps of membrane separation for increased purity. Such additional treatment can be accomplished using additional membrane separation stages to reduce atmospheric gases levels to approximately 1%, followed by cryogenic distillation. The distillation process can then separate valuable and reusable PFC compounds, such as C2F6 from the other PFC compounds and atmospheric gases to produce a high purity level, e.g., approximately 99.999%. In many instances, this level of purity is necessary for reuse in semiconductor processing.
However, membrane and distillation separation methods require expensive equipment and may entail high operating costs. A reduction or elimination of such expensive membrane and distillation separation equipment should improve the economic viability of PFC abatement and recovery.
Under prior art inventions, membrane separation devices have been combined with cold trap condensers to provide separation of condensable components from atmospheric gases. For example, U.S. Pat. Nos. 5,779,763 (Pinnau et al.); U.S. Pat. No. 5,199,962 (Wijmans); U.S. Pat. No. 5,089,033 (Wijmans); U.S. Pat. No. 5,205,843 (Kaschemekat et al.); and, U.S. Pat. No. 5,374,300 (Kaschemekat et al.), all assigned to Membrane Technology and Research, Inc. (MTR) of Menlo Park, Calif., teach methods for separating or recovering condensable components from a gas stream using a condensation step in combination with one or more membrane separation stages. U.S. Pat. No. 5,779,763 (Pinnau et al.) in particular teaches methods for separating perfluorinated compounds, including perfluorocarbons, from atmospheric gas streams.
Separation by condensation methods requires the gas stream to be cooled to the dew point temperature of the condensable components to effectuate condensation of these components. The dew point temperature for the mixture tends to increase as the pressure of the gas mixture increases. In order to reduce cooling requirements, the processes discussed in the MTR patents all involve condensation at temperatures above xe2x88x92100xc2x0 C. (xe2x88x92148xc2x0 F.). U.S. Pat. No. 5,779,763 (Pinnau et al.) which is specifically directed towards recovery of PFCs from semiconductor tool effluent streams, claims condensation at temperatures above xe2x88x9230xc2x0 C. (xe2x88x9222xc2x0 F.). These temperatures are considerably above the normal condensation temperatures of pure PFC compounds. In order to accomplish condensation of PFCs at this relatively high temperature, the gas stream must first be compressed to pressures as high as 500 p.s.i.a. Because the processes discussed in the MTR patents require relatively high working pressures to obtain condensation, the MTR processes require substantial compression equipment thus increasing operating costs.
It is desired to provide a process for reclaiming and separating condensable components from gas streams that is efficient.
It is further desired to provide a process for reclaiming and separating condensable components from gas streams that results in a high percentage of the condensable component being reclaimed.
It is further desired to provide a process for reclaiming and separating condensable components from gas streams that reduces emission of harmful gases to the environment.
It is further desired to provide a process for reclaiming and separating condensable components from gas streams that operates at cryogenic temperatures and low pressures.
It is further desired to provide a process for reclaiming and separating condensable components from gas streams that utilizes cold trapping.
It is further desired to provide a process for reclaiming and separating condensable components from gas streams that utilizes cold trapping in combination with one or more membrane separation devices and/or adsorption devices.
The present invention is a process for separating perfluorocarbon compounds from a gas mixture. A first embodiment of the process comprises multiple steps. The first step is the passing of an incoming stream of a gas mixture into a cold trap, the gas stream including a plurality of perfluorocarbon compounds. The gas mixture is cooled within the cold trap to a temperature below xe2x88x92100xc2x0 C. to produce a condensate that is enriched in at least one perfluorocarbon compound and a non-condensed gas stream from which the condensate was separated. The perfluorocarbon compound enriched condensate is then withdrawn from the cold trap. The condensate may be withdrawn by warming the cold trap to vaporize the condensate and thereafter flowing the vaporized condensate into a storage vessel. The non-condensed stream may be vented to the atmosphere, re-circulated into the cold trap or flowed through subsequent separation processes to extract additional perfluorocarbon compounds.
In a variation of the first embodiment, the cooling step is carried out within the cold trap at a temperature of approximately xe2x88x92173xc2x0 C. and a pressure of approximately 2300 torr.
In another variation of the first embodiment, the cooling step is carried out at a pressure between 1 torr and 2300 torr.
In another variation of the first embodiment, the cooling step is carried out at a temperature between xe2x88x92100xc2x0 C. and xe2x88x921900xc2x0 C.
In another variation of the first embodiment, the cooling step is carried out by thermally contacting the cold trap with a coolant medium. The coolant medium could be a cryogen. The coolant medium could be in a closed cycle cryogenic system or in an open system.
In another variation of the first embodiment, the cooling step is carried out at a preferred temperature of approximately xe2x88x92176xc2x0 C.
In another variation of the first embodiment, the process comprises the further step of membrane treating the incoming stream of the gas mixture in a membrane separation step to separate atmospheric gases from the gas mixture and venting the atmospheric gases to the atmosphere to provide to the cold trap a gas mixture enriched in a plurality of perfluorocarbon compounds.
In another variation of the first embodiment, corrosive components are removed from the incoming gas mixture stream by means of scrubbing prior to flowing the gas mixture to the cold trap.
In another variation of the first embodiment, atmospheric gases are removed from the incoming gas mixture stream by means of adsorption prior to entering the cold trap.
In another variation of the first embodiment, atmospheric gases are removed from the incoming gas mixture stream by means of a membrane separation step prior to entering the cold trap.
In another variation of the first embodiment, the membrane separation step takes place in multiple stages.
In other variations of the first embodiment, atmospheric gases and condensate are removed from the incoming gas mixture stream by membrane separation steps performed prior to and following a cold trapping step. In other variations of the first embodiment of the present invention, one or more of these membrane separation steps may be in multiple stages.
In another variation of the first embodiment, condensate is removed from the cold trap by warming the condensate and flowing it through a surge vessel and compressor and into a pressurized storage vessel. In another variation of the first embodiment, the surge vessel and compressor are removed from the process flow and the condensate is heated to a temperature sufficient to provide a driving force to flow the heated condensate into a storage vessel. In another variation of the first embodiment, the cold trap is warmed by removing it from a cryogenic source or exposing it to a source of heat.
In another variation of the first embodiment, the cooling step is carried out in multiple cold traps, e.g., two cold traps. The gas mixture is cooled within the first cold trap to produce a first condensate enriched in a first perfluorocarbon compound and to produce a non-condensed stream. The non-condensed stream is then flowed to a second cold trap wherein it is cooled to produce a second condensate enriched in a plurality of perfluorocarbon compounds.
In another variation of the first embodiment, the first cold trap is cooled to a temperature of approximately xe2x88x92139xc2x0 C. and is maintained at a pressure of approximately 100 torr while the second cold trap is cooled to a temperature of approximately xe2x88x92181xc2x0 C. and is maintained at a pressure of approximately 100 torr.
In another variation of the first embodiment, the first cold trap is cooled to a temperature of approximately xe2x88x92154xc2x0 C. and is maintained at a pressure of approximately 100 torr and the second cold trap is cooled to a temperature of approximately xe2x88x92181xc2x0 C. and is maintained at a pressure of approximately 100 torr.
In a second embodiment of the present invention, the cold trap is utilized in a two-step process. In a first step a gas mixture is introduced into a cold trap to produce a condensate. In a second step, pressure is reduced within the cold trap to withdraw from the condensate a first set of perfluorocarbon compounds having a lower boiling point and to retain within the cold trap a second set of perfluorocarbon compounds having a higher boiling point. After the pressure reduction step is completed, the retained condensate may be removed from the cold trap.
In a third embodiment of the present invention, the cold trap is utilized in another two-step process. Under this two-step process, in a first step, a gas mixture is introduced into a cold trap to produce a condensate, the condensate including a first set of perfluorocarbon compounds having a lower boiling point and a second set having a higher boiling point. In a second step, the cold trap is warmed to a predetermined temperature higher than the boiling point of the first set of perfluorocarbon compounds but lower than the boiling point of the second set to withdraw from the condensate the first set of perfluorocarbon compounds and to retain the second set within the cold trap. After the first set of perfluorocarbon compounds has been withdrawn, the retained perfluorocarbon compounds may be removed from the cold trap.
The present invention also includes several apparatuses for separating perfluorocarbon compounds from a gas mixture. A first apparatus includes a means for passing an incoming stream of a gas mixture into a cold trap, the gas mixture containing a plurality of perfluorocarbon compounds. The first apparatus also includes means for cooling the gas mixture to a temperature below xe2x88x92100xc2x0 C. so that a condensate is produced, the condensate being enriched in at least one perfluorocarbon compound. The apparatus also includes a means for withdrawing from the cold trap the condensate enriched in the at least one perfluorocarbon compound. The remaining two apparatuses perform the cold trapping process in two different two-step approaches, one using a vacuum, the other using heating.